To extract the underground resources, there has now been widely employed a method that is based on the hydraulic fracturing method. According to this method, as also disclosed in a patent document 1, an ore chute formed by drilling using a drilling machine is filled with a fluid. The fluid is then pressurized to form cracks in the ore chute in order to extract the underground resources such as petroleum (oils) and gases through the cracks. This method is also called hydraulic fracturing method, and the fluid used in this method is also called fracturing fluid.
According to this method, cracks that are formed contribute greatly to increase the sectional area of the ore chute in which the resources flow and enable the underground resources to be efficiently extracted. Therefore, this method has been widely employed for extracting, specifically, the shale gases that are produced from the sedimentary rocks that are present in relatively shallow places in the ground.
Here, in the hydraulic fracturing method, a preliminary blasting called perforation is executed in the horizontal ore chute prior to forming the cracks by pressurizing the fluid. Due to the preliminary blasting, there are formed relatively large cracks as well as a number of small cracks deep in the ore chute. Thereafter, the fluid is flown with pressure into the ore chute. Namely, the fluid flows into the cracks thereby exerting load to the cracks; i.e., the cracks grow into sizes large enough for suitably extracting the resources.
By using the fracturing fluid, the hydraulic fracturing method forms the cracks as described above and extracts the resources or gases through the cracks. Here, the hydraulic fracturing method often uses a hydrolysable material to temporarily close the cracks.
For instance, there is often used a diverting agent for temporarily closing part of the cracks that have been formed already. Namely, while closing part of the cracks that have been formed already with the diverting agent, the fluid filled in the ore chute is pressurized. The fluid, therefore, infiltrates into other cracks which then grow large in sizes. Thus a number of large cracks can be effectively formed. As the diverting agent, there is used a hydrolysable material. This is because the diverting agent must undergo the decomposition with the passage of time after it has temporarily closed the cracks.
Patent documents 1 and 2 are disclosing the use of a powder of polylactic acid and a fiber of polylactic acid as the diverting agents. Such diverting agents undergo the hydrolysis and extinguish with the passage of time. Therefore, the diverting agents do not remain in the ground, do not cause environmental contamination and do not, either, hinder the extraction of the resources such as gases and oils.
Further, in order that the cracks are not collapsed by the pressure in the ground, a crack support material is often filled in the cracks that are formed by pressurizing the fluid filled in the ore chute. The crack support material is also called proppant. As the proppant, there is usually used a granular material such as sand or the like. Generally, however, the fluid (fracturing fluid) is used in a state in which proppant particles are dispersed being applied with pressure. Thus proppant particles are forcibly introduced into the cracks that are formed.
A patent document 3 discloses the use of the polylactic acid in a fibrous form as an agent for transporting the proppant Namely, the patent document 3 teaches that the polylactic acid in the fibrous form is also a hydrolysable material which is capable of infiltrating together with the proppant into the cracks and, after the passage of a predetermined period of time, undergoes the hydrolysis and extinguishes without, therefore, remaining in the ground and effectively avoiding the cause of environmental contamination. Further, as the polylactic acid in the fibrous form enters with pressure into the cracks and, thereafter, undergoes the hydrolysis, a channel-like structure is formed in the cracks securing passages for the gases and oils to flow out. It is, therefore, made possible to efficiently extract the resources.
Further, the powder or the fiber of the polylactic acid which is the above-mentioned hydrolysable material is often used as the proppant that is a material for supporting the cracks.